Caps are known provided with a bottom, a threaded side wall intended to be screwed on a container neck, and a primary annular sealing skirt protruding from the bottom and positioned radially inside the side wall, said skirt being intended to bear radially against a cylindrical inner wall of the neck of the container. According to document WO 98 035 881 by the applicant, this inner side wall may include an annular contact bead with the neck, said bead being situated in a free end portion of the sealing skirt and turned radially outward to allow good control of the point of contact with the neck. So as also to control the contact pressure between the skirt on the neck, it is possible to give the skirt a generally tapered shape, flaring from the bottom toward the free end, as described in application WO 2005/077777. It is also possible to provide for curving the end of the skirt, as suggested in document FR 2,900,908.
The sealing obtained with this type of cap is excellent, but the contact line ensuring sealing is situated inside the neck, away from the upper rim of the neck. The threaded outer side wall and the bottom of the cap also provide protection for the assembly of the end of the neck from dust or contaminants. Under certain extreme storage conditions, however, it has been observed that contaminants or pollution may propagate from the outside, passing through the threaded interface between the cap and the neck, as far as the rim thereof.
To combat these outside contaminations, it is possible to position one or more secondary annular sealing skirts radially between the inner face of the side wall and the primary sealing skirt and intended to bear on the rim of the neck the container, as proposed in document EP 1,600,394. However, when the cap is completely molded from plastic material, it is particularly delicate to dimension the secondary skirts so as to give them a sufficient elasticity to obtain the desired barrier effect. This type of fine secondary skirt is well suited to smooth necks, but does not accommodate necks with an imperfect geometry very well.
In application WO 08 012 426 it was proposed, to improve the sealing of a threaded covering cap traditionally including a bottom, an inwardly threaded side wall and a tamperproof strip at the end of the side wall, to combine several sealing means, and in particular: an inner annular sealing skirt protruding from the bottom of the cap to come into contact with an inner cylindrical wall of the neck, an outer annular sealing skirt, positioned radially outside the inner sealing skirt, protruding from the bottom of the cap to ensure contact with an outer wall of the rib of the neck of the container, an additional annular skirt positioned axially between the inner thread of the side wall of the cap and the tamperproof strip, protruding radially toward the inside and axially toward the tamperproof strip, the additional skirt being intended to bear elastically against a collar of the neck of the container, and an annular seal covering the portion of the bottom situated between the inner skirt and the outer skirt, and which bears on the upper rim of the neck of the bottle. It is provided that the seal, preferably made from thermoplastic elastomer (TPE), can be applied either a posteriori on a joiner machine, or bi-injected with the cap. However, no bi-injection method is described and, in practice, the bi-injection of such a cap poses technical problems.
Also known, in another context, are bi-material caps including a first portion made from a plastic material forming a bottom, a side wall protecting a container neck, and a second portion made from a flexible material, in particular thermoplastic elastomer (TPE), compatible with the plastic material of the first portion, that second portion constituting an annular sealing skirt positioned radially inside the side wall. In document EP 1,373,086, a technique is proposed for attaching such a seal on the bottom of a cap, by pouring or molding the material of the seal on the bottom provided with catching protrusions for example having a dovetail profile. The control of the bi-injection method for such caps is delicate. To prevent the flexible material making up the seal from being driven during injection of the plastic material forming the first portion of the cap, it is necessary to wait for the seal to cool.